1. Field of the Invention
This invention relates to a method of correcting one or more errors in cross-interleaved data to which two error correction codes C.sub.1 and C.sub.2 are added.
2. Description of Related Art
A compact disk (CD) employs a cross-interleaved Reed Solomon code (hereafter referred to simply as CIRC) for error correction. A general construction of a CIRC decoder for a compact disk is shown in FIG. 7. Referring to FIG. 7, data of 32 symbols for one frame read out from a compact disk and EFM demodulated are supplied to a descramble circuit 21, in which odd-numbered symbols of the data are delayed by one frame interval. The output data of the descramble circuit 21 are inputted to a C.sub.1 decoder 22.
The C.sub.1 decoder 22 performs error correction of the data of 32 symbols of one frame with 4 symbols for P parity and outputs data of 28 symbols of one frame. The data of 28 symbols of one frame are supplied to a deinterleave circuit 23, in which deinterleaving over the maximum of 108 (27D) frames is performed. The output data of the deinterleave circuit 23 are supplied to a C.sub.2 decoder 24.
The C.sub.2 decoder 24 performs error correction of the data of 28 symbols of one frame with 4 symbols for Q parity and outputs data of 24 symbols of one frame. The data of 24 symbols of one frame after the error correction process are supplied to another descramble circuit 25, in which re-arrangement of order and one-frame delaying of the data are performed to restore the original order of the data. The output data of the descramble circuit 25 are supplied to a processing circuit at a next stage such as, for example, an interpolation circuit.
Upon such error correction of the CIRC as described above, error pointers each representative of presence or absence of an error are produced upon composite decoding of C.sub.1 and C.sub.2 codes and transmitted to the succeeding stages to be utilized for error correction or interpolation in order to assure a high degree of reliability in the error correction process.
In conventional error correction techniques as described above, an error pointer is produced for each symbol, constituting a frame. Consequently, the conventional error correction is disadvantageous in that a large number of error pointers is required; thus, requiring a corresponding large capacity of RAM (random access memory, hereinafter referred to as "pointer RAM") for storing error pointers therein. In addition, a large number of access times to the pointer RAM for writing and reading is required and much time is required for processing the pointers. The disadvantage is particularly significant with a CD-ROM (compact disk read only memory) in which high speed reproduction such as quadruple speed reproduction can be performed.